Ballast water means seawater that is supplied in a vessel to maintain balance of the vessel when the vessel is anchored in a certain harbor to be unloaded or sails without being loaded. In the case where the vessel filled with the ballast water is anchored in a harbor of another country and then is loaded, the ballast water in the vessel should be discharged to the harbor of the country. However, marine organism species contained in the seawater may cause damage to a marine ecosystem of another country. Therefore, it is necessary to treat ballast water in the vessel before discharging the ballast water to the sea of another country.
FIG. 1 is a sectional view showing a conventional device for filtering ballast water (hereinafter, referred to as a conventional ballast-water filtering device) using a filter, which is configured to discharge backwash water through a lower portion of the filtering device.
Referring to FIG. 1, the conventional ballast-water filtering device is configured to perform filtering by circumferentially installing a conical filter a in a cylindrical body g of the filtering device, introducing ballast water (hereinafter, referred to as original ballast water) taken from the sea through a lower end of the filtering device and passing the ballast water through the filter a in the body g of the filtering device. Further, this is configured such that, if a rotating shaft b2 passing through the body g of the filtering device and a suction portion c provided on a lower portion of the filtering device to be coupled with the rotating shaft are rotated via a motor b1 disposed on an upper portion of the filtering device to make the suction portion c communicate with the filter a, a differential pressure is generated between the suction portion c and the inside of the filtering device, so that filtered ballast water (hereinafter, referred to as filtered water) in the filtering device is introduced into the filter to wash foreign substances of the filter off and ballast water (hereinafter, referred to as backwash water) containing foreign substances flows through the suction portion c and is discharged through a discharging portion d.
However, the filtering device is problematic in that, when backwash water is accumulated in the discharging portion d, back pressure is created within the discharging portion d. This results in an increase in pressure within suction portion c is and a consequent creation of a differential pressure between the suction portion c and the filter a. Thus, the filter a is not smoothly back-washed by the backwash water; as a result of which filtering efficiency is deteriorated.
Further, the conventional ballast-water filtering device should be configured such that the motor b1 is located on the upper portion of the filtering device, so the rotating shaft b2 passes through the inside of the body g of the filtering device so as to rotate the suction portion c provided on the lower portion of the filtering device. Thus, the rotating shaft b2 occupies a considerable space, so that a space in the filtering device is reduced and thereby it is difficult to accommodate a lot of filters a. Consequently, such a ballast-water filtering device is unsuitable for use in a marine environment, which requires high filtering efficiency.
In this case, since portions on both upper and lower surfaces of the filtering device through which the rotating shaft b2 passes require air-tightness, a structure is complicated, sealing cost is increased, and the original ballast water may be mixed with the filtered water when the sealing is defective. If the filtering device is configured such that the rotating shaft b2 of the motor b1 passes through the filtering device, it is difficult to keep the balance of the rotating shaft b2 during the installation of the filtering device.
The conventional ballast-water filtering device is problematic in that the motor b1 is provided on the upper portion of the filtering device, so that the entire height of the filtering device should be increased and thereby the filtering device occupies a significant amount of space when installed in the vessel. Further, this ballast-water filtering device is problematic in that an operation of removing the motor b1 from the upper portion should be preceded when the inside of the filtering device is repaired or its components are replaced with new ones, thus making it difficult to carry out repair or replacement.
Moreover, the conventional ballast-water filtering device is problematic in that, when the backwash water of the suction portion c flows along the rotating shaft, perfect sealing is not achieved at a position where the rotating shaft b2 passes through the filtering device, so that the backwash water leaks out of the filtering device.